Investigation of Pore Pressure Migration During Piezocone Tests

在岩土工程、环境工程和其他与地下水相关的领域中，压电式锥体耗散测试是一种测量锥体通过土柱的方法。该试验通常用于表征水在土壤中渗透的难易程度(即土壤渗透性)，这是进行工程分析所必需的，如预测土壤固结沉降率或预测地下水渗漏引起的污染物迁移。然而，由于几个主要的简化假设，解释这些测试的方法是有限的。测试的方法是推进圆锥体穿过土柱，测量圆锥体肩部的孔隙水压力，然后暂停贯入并记录孔隙水压力如何变化。在饱和土体中，贯入锥体周围的土体变形会产生超孔隙水压力场。然后，当渗透暂停时，随着时间的推移，在耗散曲线中，当多余的孔隙水压力消散到流体静力状态时，就会被记录下来。现有的由耗散曲线解释土壤渗透性的方法假定：(I)超孔隙水压力的迁移是水平的，(Ii)测得的孔隙水压力衰减到静水状态是单调的。然而，有一组重要的非单调压电锥耗散试验数据，在这些试验中，测得的超孔隙水压力最初增加，然后下降到静水条件，这可能是由于垂直方向上的超孔隙水压力迁移所致。本项目将调查和表征非单调压电锥耗散试验的原因，这将导致改进的压电锥耗散解释方法，并根据试验数据更准确地表征土体的渗透性。该项目还包括在K-12工程外展活动中的演示，以增加俄勒冈州波特兰小学学生对土木工程概念的参与。该项目的目标是用数值压电锥模型表征导致非单调压电锥耗散曲线的机制。该项目假设垂直和水平的超孔隙水压力迁移对非单调的压电锥耗散曲线有贡献。还假设这些贡献受土壤应力历史(即超固结比)、土壤性质、水力传导各向异性和渗透排水条件的影响。利用直接轴对称贯入模型和MIT-S1本构模型对饱和粘土中压电锥的耗散进行数值模拟，验证这些假设。模拟将研究：(I)土壤应力历史和土壤性质对贯通锥体周围的超孔隙水压力分布和随后的耗散曲线的影响，(Ii)垂直和水平孔隙水压力迁移对耗散曲线的影响，(Iii)水力传导各向异性对垂直和水平孔隙水压力迁移的影响，以及(Iv)上述因素如何在渗透过程中受部分排水条件的影响。这些分析的结果将被综合起来，以建议非单调耗散测试的解释方法，并重新解释现有的非单调耗散测试。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The piezocone dissipation test, where an instrumented cone is advanced through the soil column, is used throughout geotechnical engineering, environmental engineering, and other groundwater-related fields. The test is routinely used to characterize how easily water seeps through soil (i.e. soil permeability) which is necessary for engineering analyses such as predicting rate of soil consolidation settlements or predicting contaminant transport by groundwater seepage. However, methods for interpreting these tests are limited due to several major simplifying assumptions. The test is performed by advancing the cone through the soil column and measuring porewater pressures at the cone shoulder, then pausing penetration and recording how porewater pressures change. Soil deformations around the penetrating cone induce an excess porewater pressure field in saturated soil. Then when penetration is paused, excess porewater pressures are recorded over time as they dissipate to hydrostatic conditions in a dissipation curve. Available methods to interpret soil permeability from dissipation curves assume that: (i) excess porewater pressure migration is horizontal, and (ii) measured porewater pressure decay to hydrostatic conditions is monotonic. However, there is a significant dataset of non-monotonic piezocone dissipation tests, where measured excess porewater pressure initially increases then decreases to hydrostatic conditions, which may be due to vertical excess porewater pressure migration. This project will investigate and characterize the causes of non-monotonic piezocone dissipation tests, which will lead to improved piezocone dissipation interpretation methods and more accurate characterization of soil permeability from the test data. The project also includes demonstrations at K-12 engineering outreach events to increase the engagement of Portland, Oregon grade school students with civil engineering concepts.The objective of this project is to characterize the mechanisms that contribute to non-monotonic piezocone dissipation curves with a numerical piezocone model. This project hypothesizes that vertical and horizontal excess porewater pressure migration contribute to non-monotonic piezocone dissipation curves. It is also hypothesized that those contributions are affected by soil stress history (i.e. over-consolidation ratio), soil properties, hydraulic conductivity anisotropy, and penetration drainage conditions. These hypotheses will be tested with numerical simulations of piezocone dissipation in saturated clay using a direct axisymmetric penetration model and the MIT-S1 constitutive model. The simulations will investigate: (i) the role of soil stress history and soil properties on excess porewater pressure distribution around the penetrating cone and subsequent dissipation curves, (ii) the role of vertical and horizontal porewater pressure migration on dissipation curves, (iii) the impact of hydraulic conductivity anisotropy on vertical and horizontal porewater pressure migration, and (iv) how the above factors are affected by partial drainage conditions during penetration. The results of these analyses will be synthesized to suggest interpretation methods for non-monotonic dissipation tests and to re-interpret existing non-monotonic dissipation tests.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.